Catalysts are available for promoting catalytic combustion, i.e., reactions between hydrocarbons or hydrogen and oxygen and between nitrogen oxides and hydrocarbons or hydrogen. These catalysts are useful in eliminating nitrogen oxides from gases which also contain some oxygen. In general, reaction of oxygen with combustible hydrocarbons or hydrogen provides sufficient heat to raise the temperature of the gas mixture so that nitrogen oxides present will decompose when the oxygen present is depleted by combustion. Reactions which occur, among others, in the catalyst are: EQU CH.sub.4 +2O.sub.2 --CO.sub.2 +2H.sub.2 O (1) EQU CH.sub.4 +4NO--2N.sub.2 +CO.sub.2 +2H.sub.2 O (2) EQU CH.sub.4 +2NO.sub.2 --N.sub.2 +CO.sub.2 +2H.sub.2 O (3) EQU H.sub.2 +O.sub.2 --2H.sub.2 O (4) EQU 2H.sub.2 +2NO--N.sub.2 +2H.sub.2 O (5) EQU 4H.sub.2 +2NO.sub.2 --N.sub.2 +4H.sub.2 O (6)
The reactions (1) through (6) above are all exothermic and provide considerable heat. Reactions between oxygen and hydrocarbon gases or between oxygen and hydrogen initiate at different temperatures. Generally, hydrogen will react at about 450.degree. F. and methane at approximately 850.degree. F. while propane will react at some intermediate temperature.
Many chemical plant heat balances such as that for nitric acid plants require that temperatures out of the abatement catalyst be high and nitrogen oxides low. Plant efficiencies are markedly reduced and pollution limits for nitrogen oxides exceeded when catalysts do not function. In this case not only is expensive hydrocarbon wasted but power is lost either because of decreased steam generation or because of lower temperature gas to power recovery turbines.
In nitric acid plants, the tail gases usually contain an objectionable amount of nitrogen oxides which constitute an atmospheric pollutant. It is well known in the art of nitric acid production to purify the tail gases by employing a process in which natural gas is used in catalytic combustion of the tail gases but since the operating temperature of noble metal catalysts should be below 1600.degree. F., it is the practice in such a process to employ two catalytic reactors in series with an intermediate waste heat boiler to lower the process temperature. In that process the purified tail gases are frequently cooled by still another waste heat boiler before the energy in the tail gases is recovered in a turbine.
The above described process employing natural gas, hereinafter referred to as prior art process A, may be summarized as entailing, in general, the heating of the cold tail gas per se, which is derived from an absorption tower in which nitric acid is produced, followed by the mixing of natural gas principally containing methane with the hot tail gas. The resulting gas mixture, now at a temperature of about 900.degree. F., which is a temperature above the initiation temperature for the catalytic reactions (1), (2) and (3), shown supra, is passed in contact with a noble metal catalyst which promotes these exothermic reactions. The resulting process gas stream, now at a more highly elevated temperature which is below 1600.degree. F., is cooled in a waste heat boiler and then passed again through a second catalytic reactor to complete the reactions (1), (2) and (3). The resulting very hot purified tail gas is cooled in a second waste heat boiler prior to passage to a turbine for the recovery of energy, prior to discharge to atmosphere. Process A has several disadvantages, the most important being (1) high initial tail gas temperature requirement to promote catalytic combustion (about 900.degree. F.), and (2) high cost of equipment. In a typical application of the prior art process A, the material balance is:
TABLE 1 ______________________________________ Tail Gas Natural Gas Combusted Gas ______________________________________ Mol % N.sub.2 95.5 94.09 O.sub.2 2.6 A 1.0 0.98 NO.sub.X 0.3 H.sub.2 O 0.6 3.44 CH.sub.4 100.0 0.07 CO.sub.2 1.42 Total 100.0 100.0 100.0 Lbs./Hr. 118,800 1026 119826 Mols/Hr. 4220 64.1 4390.8 ______________________________________
It is also well known in the art of nitric acid production to purify the tail gases by employing a process in which purge gas is used in catalytic combustion of the tail gases thus permitting a relatively low operating temperature for the catalyst (about 1300.degree. F.) but those purified gases still have to be cooled before the energy is recovered in a turbine.
The above described process employing purge gas, hereinafter referred to as prior art process B, may be summarized as entailing procedural steps comparable to process A described supra, except that a hydrogen-containing gas is mixed with the tail gas and catalytic reactions (4), (5) and (6) are carried out instead of reactions (1), (2) and (3). The initiation temperature for catalysis in process B is generally about 630.degree. F., which is lower than for process A, and a lower temperature of about 1300.degree. F. is attained by the catalysis. A typical hydrogen-containing gas employed in process B is the purge gas from an associated ammonia synthesis process. This purge gas contains hydrogen and nitrogen in an approximately 3:1 molar ratio together with inerts such as argon and methane, as shown in the following material balance. Process B has several disadvantages, the most important being (1) high initial tail gas temperature requirement to promote catalytic combustion (about 630.degree. F.), (2) high cost of equipment, and (3) frequent scarcity of suitable purge gas in most nitric acid plants. In a typical application of the prior art process B the material balance is:
TABLE 2 ______________________________________ Tail Gas Purge Gas Combusted Gas ______________________________________ Mol % N.sub.2 95.5 21.3 93.20 O.sub.2 2.6 A 1.0 4.0 1.17 NO.sub.X 0.3 H.sub.2 O 0.6 4.96 CH.sub.4 12.9 .07 CO.sub.2 0.1 .60 H.sub.2 61.7 Total 100.0 100.0 100.0 Lbs./Hr. 118,800 2468 121268 Mols/Hr. 4220 226.67 4383.02 ______________________________________